Is there a C# unit test framework that supports arbitrary expressions rather than a limited set of adhoc methods?

Question

Basically NUnit, xUnit, MbUnit, MsTest and the like have methods similar to the following:

Assert.IsGreater(a,b) //or, a little more discoverable Assert.That(a, Is.GreaterThan(b)) 

However, there are a limited number of such comparison operators built-in; and they duplicate the languages operators needlessly. When I want anything even slightly complex, such as...

Assert.That(a.SequenceEquals(b)) 

I'm often either left digging through the manual to find the equivalent of the expression in NUnit-speak, or am forced to fall-back to plain boolean assertions with less helpful error messages.

C#, however, integrates well with arbitrary Expressions - so it should be possible to have a method with the following signature:

void That(Expression<Func<bool>> expr); 

Such a method could be used to both execute the test (i.e. validate the assertion) and to also provide less-opaque diagnostics in case of test failure; after all, an expression can be rendered to pseudo-code to indicate which expression failed; and with some effort, you could even evaluate failing expressions intelligently to give some clue of the value of subexpressions.

For example:

Assert.That(()=> a == b);//could inspect expression and print a and b Assert.That(()=> a < b && b < c); //could mention the values of "a<b" and "b<c" and/or list the values of a, b, and c. 

At a minimum, it would make the use of a parallel language for expressions unnecessary, and in some cases it might make failure messages more useful.

Does such a thing exist?

Edit: After trying (and liking!) Power Assert, I ended up reimplementing it to address several limitations. My variant of this is published as ExpressionToCode; see my answer below for a list of improvements.

Answer 1

Check out the PowerAssert library (example output below):

PAssert.IsTrue(() => x + 5 == d.Month * y);   System.Exception : IsTrue failed, expression was:  x + 5 == d.Month * y | |   |  | |     | | | |   |  | |     | 6 | |   |  | |     18 | |   |  | 3 | |   |  01/03/2010 00:00:00 | |   False | 16 11 

http://powerassert.codeplex.com/